The present invention is related to apparatus for determining the optical powers of a test lens and more particularly concerned with the apparatus for converting signals indicative of the net refractive power of a test lens along each of its orthogonal meridians to a so-called prescription form comprising a sphere power and a cylinder power.
Literally millions of lenses are manufactured in the United States annually. A large percentage of these lenses are ophthalmic lenses for use in spectacles. Since these ophthalmic lenses are intended to correct and/or protect the vision of the wearer, the lenses must be carefully examined and accurately measured in order to meet the wearer's needs, particularly the refractive power along each of the two principal meridians and the angular orientation of a referenced one of the principal meridians.
Various lens measuring instruments have been used to provide the lens parameter data discussed above. A general type of such instrument is known as a focimeter, the basic principle of which has been long known, as evidenced in Troppman U.S. Pat. No. 1,083,309 issued Jan. 6, 1914. These instruments operate by determining the focal length of a test lens along its two principle meridians when placed in a particular optical system. The determined focal lengths accordingly correspond with the net refractive powers of the lens along the respective principle meridians. Most of these instruments have been, and are, manually operated and employ a subjective determination of the "focus" condition, although instruments have been recently developed which are capable of determining lens power automatically and objectively, as typified in U.S. application Ser. No. 703,492 filed July 8, 1976.
Even though the basic parameters of the test lens measured by the aforementioned instruments are its net refractive powers along the respective principal meridians, it has long been the practice of doctors and others associated with eye care to express a patient's needs in a so-called prescription form. This prescription form specifies a "sphere power" and a "cylinder power" and also the angle of the so-called cylinder meridian (axis) relative to the horizontal.
The doctor may express a patient's prescription in either a so-called "+ cylinder" from in which the "cylinder" component of power is always expressed as "+", or a so-called "- cylinder" form in which the "cylinder" component is expressed as "-". The particular form used will depend on the doctor and the refraction equipment used. The prescribed cylinder power is thus correspondingly additively or subtractively referenced to the sphere power, and its (cylinder) meridian or axis angle is expressed relative to the horizontal. However, the lens manufacturer and/or supplier may work with a system in which the cylinder component of power is expressed in the oppositely-signed form from that expressed in a particular doctor's prescription.
The various available aforementioned focimeters have heretofore only provided a direct indication of the net refractive power along each of the principal meridians of a test lens relative to some predetermined zero reference, and the translation of those net refractive powers values into the so-called "prescription form" has typically required an operator to determine the magnitude and the sense or sign of the difference between the first and second net power readings in order to provide a signed cylinder component. Furthermore, because the doctor's prescription and the lens supplier's prescription may be expressed in oppositely-signed cylinder notation, it was then necessary to mentally convert from one form to the other by changing the sign of the cylinder component and modifying the value of the sphere component by arithmetic addition of the cylinder component therewith, as well as also modifying the indicated axis angle by 90.degree.. It will be appreciated that the need to convert net refractive power readings to one, and possibly also the other, prescription form (plus and/or minus cylinder notation) is both time consuming and introduces the opportunity for operator error.
Accordingly, the present invention has as its principal objective the provision of apparatus for determining the net refractive powers of a test lens along each of its respective principal meridians and at least semi-automatically expressing those optical characteristics of the lens in prescription form.
It is a further object of the present invention to provide apparatus for simply, quickly and accurately converting lens power characteristics expressed in prescription form from either the plus or minus cylinder notations to the oppositely signed cylinder notation.
It is a still further object of the invention to provide apparatus for indicating the appropriate cylinder axis angle associated with a particular respective lens power prescription.